The only times I've trouble with BT outside is when tons of people or shops are nearby (overcrowded 2.4 ghz spectrum I assume - bt, wifi, etc)

Not sure. I remember reading an article some time ago that stated the Sun's electromagnetic radiation could interfere with Bluetooth and it's always worked with my observations. I've used multiple Plantronics headsets, Motorola headphones and an off brand of headphones all with an iPhone 5S, iPhone 6+, Nexus 6P, Moto X and a Note 3 and in all of these devices they work great indoors and going outside on a sunny day it would break up (but cloudy always seemed to work better).

It's entirely possible this is some crazy bullshit but my observations are real. So if it ends up being that the Sun does NOT provide interfere then it must be SOMETHING. I just had this issue this past week when I was working outside and had to grab my wired headphones.

And likely your best results will be if both devices are v5 devices.

Well, I've used wireless keyboards (Apple compact) and wireless headphones (Parrot Zik 2.0), and had zero disconnects under the hottest of Suns in several different countries.

What do you mean "line of sight"? How far were you? Was it behind some wall or something? With the iPad/phone on my pocket/bag I never had any issue.

Basically if the BT hardware itself was on the right side of my head then my phone had to be in my right pocket or I'd get semi frequent disconnects outside. So not really line-of-sight in visual spectrum terms but I have had so many observations with so many bluetooth headsets and headphones and different phones that I always believed the original article I read a long time ago about sun interference (which, of course, I can't find today).

It could certainly be something else but I most certainly have reproducible issues with using bluetooth outside versus inside my house or even car.

Sounds like it's more about some flimsy and/or underpowered BT headsets.